a. Field of the Invention
The present invention relates to a polyimide-based friction material which is excellent in heat-resistance, abrasion resistance, friction characteristics and mechanical properties and to a process for preparing the friction material. The polyimide-based friction material of the invention can be widely utilized as friction parts in any fields, for example, automobiles, office machines and electric and electronic equipments.
b. Description of the Prior Art
Compositions obtained by using synthetic resins such as phenol resin or melamine resin as a binder and by bending mineral fibers such as asbestos as a filler have conventionally been used for a component of friction material for use in brakes and clutches. The friction material composed of these compositions, however, has a disadvantage that, in braking under high speed and high load and particularly under high temperature, friction coefficient decreases due to the effect of the heat-deteriorated ingredient of the transferred resin on the surface of the opposite material and brake performance is accordingly decreased. For example, the friction material composed of the above compositions containing the phenol resin exhibits marked reduction of friction coefficient at high temperatures in the range of 150.degree. to 200.degree. C. Asbestos fibers blended as a filler is unfavorable because it flies in the surroundings with abrasion of the friction material and is liable to give adverse effects on humans.
In order to remove the disadvantages of the friction material composed of the above composition, Japanese Patent Application Laid-open No. 144363 (1985) has disclosed a friction material obtained by using aromatic polyimide resin as a binder to enhance heat-resistance and adding various inorganic fillers. In view of the effect of asbestos fibers on humans, a friction material obtained by mixing polyimide resin with potassium titanate fibers and/or processed mineral fibers and iron powder has been known as described in Japanese Patent Application Laid-Open No. 207980 (1984). Brake materials prepared from these friction materials exhibit excellent abrasion resistance under high speed and high load. However, consistency of brake performance against temperature change on the frictional surface depending upon sliding time or temperature change in the atmosphere, that is, stability of friction coefficient has not always been satisfactory.
In order to enhance mechanical strengths such as flexural strength and impact strength, abrasion resistance and critical PV value (P=frictional surface pressure, V=friction velocity), it has been known to prepare a friction material by blending a binder with inorganic fibers such as glass fiber, asbestos fiber, potassium titanate fiber and processed mineral fiber, or heat-resistant organic fibers such as aramid fiber and aromatic polyester fiber. However, the friction material obtained by blending glass fiber causes serious damage to the opposite material. Friction material obtained by blending asbestos fiber, potassium titanate fiber or processed mineral fiber gives less damage to the opposite material as compared with glass fiber. However, the damage cannot be neglected when the blended amount is increased. Friction material obtained by blending heat-resistance organic fibers is excellent in causing no damage to the opposite material. The friction material, however, has poor machinability, the finish is coarse, and the so-called surface smoothness is poor. These properties give adverse effects on the dimensional tolerance of mechanical parts and also on the time dependent variation of friction coefficient.
As to friction material without containing these fibrous fillers, Japanese Patent Application Laid-Open No. 137436 (1987) has disclosed a polyimide base friction material obtained by blending fluoro resin, transition metals and/or transition metal oxides to non-thermoplastic aromatic polyimide resin. The aromatic polyimide resin, however, cannot be processed by fusion method such as injection molding and can only be processed by hot compression molding. Consequently, manufacture of molded articles having a complex shape has very low productivity and high cost.